Conductive shields provide signal isolation in electrical circuits and systems. Typically, conductive shields are positioned on opposite sides of a printed circuit (PC) board and screwed together, sandwiching the PC board between the shields. Compressible conductive gaskets placed between the PC board and each of the shields lowers electrical resistance at each of the board-shield interfaces. Lower electrical resistance generally produces higher signal isolation. Since performance specifications in many circuits and systems are only attainable if signal isolation is sufficiently high, providing a low electrical resistance at the board-shield interfaces is critical.
The electrical resistance at each board-shield interface decreases as pressure on the gasket increases, causing signal isolation to be limited by the point along the interface at which the mechanical pressure on the gasket is lowest. Therefore, sufficiently high mechanical pressure must be applied to the compressible conductive gasket uniformly, at all points along the board-shield interface, to achieve sufficiently high signal isolation. Providing uniform mechanical pressure to the compressible conductive gaskets is a difficult task. Typically, attachment screws are used to fasten the conductive shields to the PC board, and when the screws are tightened, the shields deform. This deformation results in uneven pressure on the gasket. The pressure on the gasket in the vicinity of the attachment screw is higher than the pressure on the gasket at other points along the interface.
In presently used shielding schemes, uniform mechanical pressure is provided to the gasket by minimizing deformation of the conductive shields. Deformation is minimized by making the shields rigid and by positioning the attachment screws at closely-spaced intervals. However, making the shields rigid results in the walls of the shields being thick which causes a large contact area between the compressible gasket and the PC board. This large contact area occupies PC board area that would otherwise be useable for mounting electrical components. In addition, closely spacing the attachment screws increases the total number of attachment screws in the shielded PC board assembly, causing the assembly time and manufacturing costs of shielded PC board assemblies to correspondingly increase. Accordingly, there is a need for a PC board shielding scheme that achieves sufficiently high signal isolation while minimizing contact area and minimizing the number of attachment screws.
In a shielding scheme for circuit boards constructed according to the preferred embodiment of the present invention, conductive shields have contoured mating surfaces that become flat, or co-planar, at the circuit board-shield interface when the shields are fastened to opposite sides of the circuit board. The contoured mating surfaces compensate for deformation of the shields resulting from the fastening so that uniform mechanical pressure is applied at all designated points along the circuit board-shield interface even though fastening points are intermittently spaced throughout the shields. When compressible conductive gaskets are optionally interposed between the shields and the circuit board, stops are included to accommodate for the thickness of the gasket. High signal isolation is achieved without correspondingly high contact area on the circuit board and without closely-spaced fastening points. Low assembly time and manufacturing cost results for shielded circuit board assemblies incorporating the shielding scheme.